High-purity tellurium dioxide single crystal and manufacturing method thereof

ABSTRACT

A high-purity tellurium dioxide (TeO 2 ) single crystal and its manufacturing method are provided. The method comprises the following procedures: firstly performing a first single crystal growth, and then dissolving the resulting single crystal again, thereafter adding a precipitation agent to form powder, and finally performing a second single crystal growth of as-prepared powder to obtain the high purity single crystal. The TeO 2  single crystal prepared according to present invention is of high purity, especially with a content of radioactive impurities such as U and Th decreased to a level of 10 −13  g/g.

TECHNICAL FIELD

The present invention relates to a high-purity tellurium dioxide (TeO₂)single crystal and its manufacturing method, which belongs to thetechnical field of single crystal growth.

BACKGROUND

In the field of nuclear physics, the experiments of neutrino have drawnworld wide attention in science frontline. In 1956, Clyde Cowan andFrederick Reines directly detected the neutrino in experiment and theywere awarded the 1995 Nobel Prize in physics; in 1962, Leon M. Lederman,Melvin Schwartz and Jack Steinberger discovered a second type ofneutrino, i.e. muon neutrino and they thus wined the 1988 Nobel Prize inphysics; Raymond Davis Jr. and Masatoshi Koshiba were jointly awardedthe 2002 Nobel Prize in Physics, in which Davis for his pioneer work oncosmic neutrinos in 1968 and Koshiba for the first real time observationof supernova neutrinos in 1987. However, till now the questions aboutneutrino mass scale and the transformation between different types ofneutrinos are still basic scientific issues need to be answered innuclear physics (Zhixun Huang, Engineering Science, 2002, 4(10), 7-10).

In the research field of neutrino, the decay rate (half life) of doublebeta decay is an important issue because the relationship between decayrate (half life) and absolute mass of neutrino can be expressed asfollowing:τ_(1/2) ⁻¹ =G ^(thr)(Q,Z)|M ^(Gr)|²|(m _(v))|²

So, the absolute mass of neutrino can be known through the half lifedetection of double beta decay. Till now, double beta decay experimentsseem to be still the only way to detect the absolute mass of neutrino(R. Ardito et al., Annual Report for CUORICINO and CUORE 2004, 17-37).

Tellurium dioxide (TeO₂) single crystal is an excellent acousto-optic(AO) material, which can be used in various kinds of AO devices, such asAO deflectors, AO modulators, AO resonators and tunable filters (S.Kumaragurubaran et al., J. Crystal Growth 2000, 211, 276-280; P. Veberet al., J. Crystal Growth 2004, 270, 77-84).

Besides, TeO₂ single crystal has double beta decay property, which canbe used in nuclear physics as double beta decay source (I. Dafenei etal., Nucl. Instrum. Meth. A 2005, 554, 195-200). As the naturalabundance of ¹³⁰Te is high (34%, the highest level of all natural doublebeta decay source), TeO₂ single crystal can be used as double beta decaysource without enrichment process, thus the cost is much lower.

However, if TeO₂ single crystal is used as double beta decay source forthe detection of neutrino mass, the crystal itself should be of highpurity, especially with a content of some radioactive impurities such asU and Th deceased to a level of 10⁻¹³ g/g.

Chinese patent ZL03141999.2 reported the Bridgman growth of TeO₂ singlecrystal with large size and good quality, through which the radioactiveimpurities such as U and Th decreased to a level of 10⁻¹² g/g, but stillcouldn't reach the level required for double beta decay source.

Until now (filing date of the present invention), we haven't found anyreports about decreasing impurities to prepare high pure TeO₂ singlecrystal.

SUMMARY OF THE INVENTION

The first aim of the present invention is to provide a manufacturingmethod for high-purity TeO₂ single crystal. The method involves a doublegrowth technique, i.e. through double impurities repellence processes,purifying the crystal and accelerating the decay process of radioactiveelements, thus further decrease the radioactive impurities, and finallyget high-purity TeO₂ single crystal.

The manufacturing method for high-purity TeO₂ single crystal provided bythe present invention has characteristics as following:

-   -   (1) filling crucible with TeO₂ powder to carry out the first        single crystal growing process;        -   wherein the TeO₂ powder is commercial or self-made;        -   wherein the preparation methods for TeO₂ powder are commonly            used methods in this field, including but not restricted to            one of the methods from dissolving-neutralization method,            chlorination-neutralization method, tellurite decomposition            method or direct oxidation method;        -   wherein the crystal growth method includes Bridgman method            and Czochralski method;    -   (2) dissolving the single crystal prepared in step (1) with acid        solution;        -   wherein the acid solution could be hydrochloric acid or the            mixed acid of hydrochloric acid with other acid, in which            other acid preferably inorganic acid, further preferably            sulfuric acid or nitric acid, and the mixed volume ratio for            hydrochloric acid and other acid could be 1:0.01-10;        -   wherein the concentration of acid solution is 0.05-50 wt %;        -   wherein the acid solution is preferably hydrochloric acid,            and the preferable concentration is 0.05-40 wt %, further            preferable concentration is 0.1-38 wt %; before the process            of dissolving, it is preferably to remove the outer skin of            the single crystal obtained in step (1), and the preferable            thickness of removed outer skin is 0.05-5 mm, further            preferable thickness is 1-2 mm;    -   (3) adding basic precipitator to the solution of step (2),        followed by the filtering and drying process to obtain TeO₂        powder;        -   wherein the basic precipitator could be ammonia or urea, and            if the basic precipitator is mixed solution of ammonia and            urea, then the mixed ratio could be random;        -   wherein the basic precipitator is preferably ammonia, and            the preferable concentration is 0.05-35 wt %, further            preferable concentration is 0.1-30 wt %;    -   (4) filling crucible with the TeO₂ powder obtained in step (3)        (TeO₂ (II)) to carry out the second single crystal growing        process;        -   wherein the crystal growth method includes Bridgman method            and Czochralski method;        -   the technical conditions for Bridgman method or Czochralski            method are commonly used conditions in this field.

The second aim of the present invention is to provide a high-purity TeO₂single crystal, in which the concentration of U is lower than 9.9×10⁻¹³g/g, the concentration of Th is lower than 9.9×10⁻¹³ g/g, and theconcentration of U is preferably lower than 9.9×10⁻¹⁴ g/g.

The high-purity TeO₂ single crystal could be used as high quality AOcrystal for AO devices, especially AO deflectors, AO modulators, AOresonators, tunable filters and so on.

The advantages of manufacturing method for high-purity TeO₂ singlecrystal provided by the present invention are as following:

-   (1) the single crystal growing process itself could repel    impurities, therefore dissolving the single crystal obtained from    the first crystal growth and then precipitating it into powder, this    method could obtain purified powder with several times purer;-   (2) the double growth technique includes two repellence processes,    which could not only purify the single crystal, but also accelerate    the decay process of radioactive elements, thus further decrease the    radioactive impurities to more than ten times lower, and finally    obtain high-purity TeO₂ single crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematic drawings of the manufacturing method for thehigh-purity TeO₂ single crystal according to the present invention;

FIG. 2 shows the photo of high-purity TeO₂ single crystal from Example 1of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For illustrative purpose, representative embodiments of this inventionare described hereafter. It will be understood that the embodiments areused only for illustrating, not for restricting the scope of thisinvention. As for the experiment without the description of detailedconditions, it should follow the regular conditions or recommendedconditions from manufactory.

Unless other definition or explanation provided, all the professionaland scientific language used in the invention has the same meaningfamiliar with the technicians in this field. Besides, any similar orsame method or material can be applied in this invention.

Example 1

-   -   (1) The TeO₂ powder prepared with dissolving-neutralization        method (marked as TeO₂ powder (I)) was calcinated at 680° C. in        platinum crucibles for 12 h and then rapidly cooled to room        temperature. After that, the calcinated powder and oriented TeO₂        seed were filled into a platinum crucible, sealed and then        heated to 750° C. and kept at this temperature for 10 hours. The        growing process was then driven by lowering the crucible at a        rate of 1.0 mm per hour and rising the furnace temperature at a        rate of about 2° C. per hour. At the end of the growing process        the crucible was cooled down to room temperature at a rate of        30° C. per hour. The whole processes for the first growth were        finished after the annealing process at 680° C. for 12 h.    -   (2) 1-2 mm thickness of outer skin of the crystal obtained in        step (1) was removed by rough mechanical method, and then the        remained TeO₂ crystalline pieces were dissolved with 37.5 wt %        hydrochloric acid, forming TeCl₄ solution.    -   (3) 28 wt % concentrated ammonia was then added into the TeCl₄        solution prepared in step (2), and the obtained white        precipitate was further washed and dried, and finally even purer        TeO₂ powder (marked as TeO₂ powder (II)) was obtained.    -   (4) The pure powder obtained in step (3) was further filled into        a platinum crucible and a second growth process was executed        following the same procedure as in the step (1).

ICP-MS test was conducted for the purity test of TeO₂ powders before andafter the first growth, i.e. the purity of TeO₂ powder (I) and TeO₂powder (II) (seen Table 1).

TABLE 1 ICP-MS test results of TeO₂ powder (unit: g/g) Material U Th BiPb TeO₂ powder (I)  1.0 × 10⁻⁹  4.0 × 10⁻⁹ 3.7 × 10⁻⁸  2.2 × 10⁻⁷ TeO₂powder (II) <2.0 × 10⁻¹⁰ <2.0 × 10⁻¹⁰ 2.2 × 10⁻⁸ <1.0 × 10⁻⁷

From Table 1, we can see that the TeO₂ powder (I) used for the firstcrystal growth had radioactive impurities like U and Th at a level of10⁻⁹ g/g, other impurities like Pb 2.2×10⁻⁷ g/g. However, through onecrystal growing process and the following dissolving-neutralizationprocess with high pure hydrochloric acid and ammonia respectively, theobtained TeO₂ powder (II) had much higher purity. The concentration ofradioactive impurities like U and Th were lower than 2.0×10⁻¹⁰ g/g, andother impurities like Pb was lower than 10⁻⁷ g/g. From the aboveanalysis, it is clear that after a crystal growth process, impuritieswere repelled to some extent, thus the obtained single crystal hadhigher purity.

Another phenomenon is that radioactive impurities (like U and Th) seemto be removed more obviously than other impurities (like Bi and Pb). Theexplanation could be that during the crystal growth process, not onlythe impurities were purified, but also the decay process of radioactiveelements was accelerated. Therefore, using the purer TeO₂ powder (II) tostart the second crystal growth, through the double growth techniquei.e. two repellence processes, we can further decrease the radioactiveimpurities, and finally obtain high-purity TeO₂ single crystal (see FIG.2).

Using radioactive array detector, we know that the obtained high-purityTeO₂ single crystal had a content of radioactive impurities of U at alevel of 8.2×10⁻¹⁴ g/g and Th 1.1×10⁻¹³ g/g, which can fulfill thedemand for neutrino detection in nuclear physics.

Example 2˜3

The material selection and detailed parameters in example 2˜3 are listedin Table 2 (other parameters are the same as example 1).

Example 4

-   -   (1) The commercial TeO₂ powder was filled into a platinum        crucible, and then Oxygen gas was guided into at a rate of 750        ml per minute. When the powder was melted, kept at the        temperature for 4 hours. And then lowered the oriented seed        gradually. Tuning the surface temperature of the melt and let        the seed contacted the liquid surface near the balance of solid        and liquid. After seeding process, pulling at a rate of 1.2 mm        per hour and rotating the seed at a rate of 30 r/m. At the end        of the growing process the crucible was cooled down to room        temperature at a rate of 60° C. per hour. The whole processes        for the first growth were finished after the annealing process        at 680° C. for 12 h.    -   (2) 0.5-0.8 mm thickness of outer skin of the crystal obtained        in step (1) was removed by rough mechanical method, and then the        remained TeO₂ crystalline pieces were dissolved with 37.5 wt %        hydrochloric acid, forming TeCl₄ solution.    -   (3) 28 wt % concentrated ammonia was then added into the TeCl₄        solution prepared in step (2), and the obtained white        precipitate was further washed and dried, and finally even purer        TeO₂ powder (marked as TeO₂ powder (II)) was obtained.    -   (4) the pure powder obtained in step (3) was further filled into        a platinum crucible and a second growth process was executed        following the same procedure as in the step (1).

Example 5˜8

The material selection and detailed parameters in example 5˜8 are listedin Table 2 (other parameters could be known from the Bridgman method inexample 1 and the Czochralski method in example 4).

TABLE 2 Second First Crystal Crystal Growth Dissolving PrecipitationGrowth Growth Solvent and its Precipitator and its Growth Example methodconcentration concentration method Example 1 Bridgman 37.5 wt % HCl 28wt % NH₄OH Bridgman method method Example 2 Bridgman   15 wt % HCl 10 wt% NH₄OH Bridgman method method Example 3 Bridgman 37.5 wt % HCl 30 wt %NH₄OH Bridgman method method Example 4 Czochralski 37.5 wt % HCl 28 wt %NH₄OH Bridgman method method Example 5 Bridgman   15 wt % HCl 20 wt %NH₄OH Czochralski method method Example 6 Czochralski 37.5 wt % HCl 15wt % NH₄OH Czochralski method method Example 7 Czochralski 49.5 wt % HCland 15 wt % 15 wt % NH₄OH Czochralski method H₂SO₄, Vol % = 10:1 methodExample 8 Czochralski 5 wt % HCl and 40 wt % 5 wt % NH₄OH andCzochralski method H₂SO₄, Vol % = 1:10 20 wt % Urea, Vol % = 1:2 method

The invention claimed is:
 1. A manufacturing method for high-purity TeO₂single crystal, comprising: (1) Filling a crucible with TeO₂ powder tocarry out a first single crystal growing process and preparing a singlecrystal; (2) Dissolving the single crystal prepared in step (1) withacid solution; (3) Adding basic precipitator to the solution of step(2), followed by a filtering and drying process to obtain TeO₂ powder;(4) Filling a crucible with the TeO₂ powder obtained in step (3) tocarry out a second single crystal growing process and growing ahigh-purity TeO₂ single crystal.
 2. The method of claim 1, wherein theacid solution is hydrochloric acid or the mixed acid of hydrochloricacid with other acid.
 3. The method of claim 1, wherein the acidsolution is hydrochloric acid.
 4. The method of claim 1, wherein theconcentration of the acid solution is 0.05˜50 wt %.
 5. The method ofclaim 4, wherein the concentration of the acid solution is 0.05˜40 wt %.6. The method of claim 2, wherein the other acid is inorganic acid. 7.The method of claim 6, wherein the other acid is sulfuric acid or nitricacid.
 8. The method of claim 7, wherein the mixed volume ratio forhydrochloric acid and other acid is 1:0.01˜10.
 9. The method of claim 1,wherein the basic precipitator is ammonia or urea.
 10. The method ofclaim 9, wherein the basic precipitator is ammonia.
 11. The method ofclaim 9, wherein the concentration for basic precipitator is 0.05-35 wt%.
 12. The method of claim 9, wherein the mixed ratio for ammonia andurea is random.
 13. The method of claim 1, wherein the crystal growthmethod includes Bridgman method and Czochralski method.
 14. The methodof claim 1, wherein the TeO₂ powder is commercial or self-made, whereinthe self-made methods include one of the methods fromdissolving-neutralization method, chlorination-neutralization method,tellurite decomposition method or direct oxidation method.
 15. Themethod of claim 1, wherein the preferable thickness of removed outerskin is 0.05-5 mm.
 16. A high-purity TeO₂ single crystal, wherein theconcentration of U is lower than 9.9×10⁻¹³ g/g, Th is lower than9.9×10⁻¹³ g/g.
 17. A high-purity TeO₂ single crystal, wherein theconcentration of U is lower than 9.9×10⁻¹⁴ g/g.
 18. The high-purity TeO₂single crystal of claim 16, wherein the crystal is used in acousto-opticdevices.
 19. The high-purity TeO₂ single crystal of claim 18, whereinthe crystal is used in AO deflectors, AO modulators, AO resonators, ortunable filters.